<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div class="root">
        <div class="item"> 1 </div>
        <div class="item"> 2 </div>
        <div class="item"> 3 </div>
    </div>
</body>

<style>
    .root{
        width: 500px;
        height: 500px;
        border: 1px solid lightgray;
        margin-top: 50px;
        margin-left: 50px;
        display: flex;
        flex-direction: row;
        /* flex-direction: row-reverse; */
        /* flex-direction: column-reverse;
        flex-direction: column; */
        justify-content: flex-start;
        /* justify-content: center; */
        /* justify-content: flex-end; */
        /* justify-content: space-between; */
        /* justify-content: space-around; */
        /* justify-content: space-evenly; */
    }
    .item{
        width: 100px;
        height: 100px;
        text-align: center;
        color: white;
        font-size: 40px;
        line-height: 100px;
    }
    .item:nth-of-type(1){
        background: red;
    }
    .item:nth-of-type(2){
        background: green;
    }
    .item:nth-of-type(3){
        background: blue;
    }
</style>

</html>